VS131 Visual Neuroscience

Lateral Geniculate Nucleus (LGN)

The major targets of the retinal ganglion cells:

RETINA LGN CORTEX (“vision’)

Superior Colliculus (saccades)

Suprachiasmatic Nucleus (circadian)

Pretectum (pupilary light reflex)

Accessory Optic System (‘steadicam’)

Only about 10% of retinal ganglion cells do not terminate in the Lateral Geniculate Nucleus (LGN). (Does NOT mean that 90% do not terminate in other nuclei).

‘BLINDSIGHT’: preservation of very limited ability to perform visually guided tasks after destruction of the retina to LGN to cortex pathway, in the apparent absence of conscious perception.

Bottom line: take out the LGN-cortical system and you are for all practical purposes completely blind.

LESIONS OF THE GENICULO-CORTICAL SYSTEM CAUSE DIRECT LOSS OF VISION IN SPECIFIC PARTS OF THE FIELD OF VIEW. IMPORTANT FOR CLINICAL DIAGNOSIS.

The LGN is part of the thalamus, which itself is part of the diencephalon.

Sometimes called the “LGNd” because it is part of the Dorsal thalamus. (No directly corresponding LGNv, maybe a small oculomotor system in cats??).

The thalamus contains sensory relay nuclei for all the senses: sometimes called “the gateway to the cortex”. (Olfaction is a partial exception: some direct connections from olfactory bulb to cortex).

Don’t panic!

(Mostly F.Y.I.)

Take home message: axons in these pathways are not positioned randomly but are arranged in order.

So what does the LGN do?

Relay information from the retina to cortex...

… and nobody has been able to demonstrate conclusively that the LGN does anything else!!!

LGN relay cells have properties that are very much like those of the retinal ganglion cells that contact them.

Probably something very important – ‘gating’ of information to cortex? We’re working on it…

LGN layers are in register.

Even though inputs from the same part of the retinas are brought close together in the LGN, they do not talk to each other – they are still monocular.

(Don’t get binocular cells until you reach cortex).

1,4,6: cross the river styx.

2,3,5: stay on the same side!

Cat! Many early experiments, terminology still used.

Cat X is homologous to monkey parvo

Cat Y is homologous to monkey magno

LGN relay neurons project (I.e., send axons) to cortex.

LGN interneurons make only local connections.

There are more interneurons than relay neurons!

LGN neurons get feedback connections from cortex. (The one-way connection from retina to rest of brain is unique in the visual system).

LGN gets other inputs as well. For example: from brainstem and perigeniculate.

STILL don’t know what the LGN might be doing other than acting as a simple relay…

WARNING: There are other classes of retinal/LGN neurons than parvo/magno (“koniocellular”, “intralaminar”, etc), but their function has not been determined well enough to make it into the textbooks just yet. Be warned that 20 years from now it will probably be a different story.

“The Vision Thing”